The present invention relates generally to a ballast circuit for gas discharge lamps. More particularly, this invention relates to a ballast circuit with multiple inverters and a dimming controller for individually controlling each inverter.
Either regulating the lamp current or regulating the average current feeding an inverter are techniques for dimming fluorescent lamps with class D converters. For cold cathode fluorescent lamps (CCFLs), the pulse width modulating (PWM) technique is commonly used to expand a dimming range. The technique pulses the CCFLs at full rated lamp current thereby modulating intensity by varying the percentage of time the lamp is operating at full-rated current. Such a system can operate with a closed loop or an open loop system. The technique is simple, low cost, and a fixed frequency operation, however, it is not easily adapted to hot cathode fluorescent lamps. For proper dimming of hot cathode lamps, the cathode heating needs to be increased, as light intensity is reduced. If inadequate heating exists, cathode sputtering increases as the lamp is dimmed. Also, the lamp arc crest factor should be less than 1.7 for most dimming ranges, in order to maintain the rated lamp life. The higher the crest factor, the shorter will be the life of the lamp. The PWM method does not address these problems, and therefore so far has been limited to CCFL applications.
Class D inverter topology with variable frequency dimming has been widely accepted by lighting industry for use as preheat, ignition and dimming of a lamp. The benefits of such a topology include, but is not limited to (i) ease of implementing programmable starting sequences which extend lamp life; (ii) simplification of lamp network design; (iii) low cost to increase lamp cathode heating as the lamp is dimmed; (iv) obtainable low lamp arc crest factor; (v) ease of regulating the lamp power by either regulating the lamp current or the average current feeding the inverter; and (vi) zero voltage switching can be maintained by operating the switching frequency above the resonant frequency of the inverter.
Conventional class D circuits which are used for d.c.-to-d.c. converters or electronic ballasts, implement a two-pole active switch via two, n-channel devices or n-p-channel complementary pairs. A gate is voltage controllable from a control-integrated circuit (IC), which is normally referenced to ground, thus, the control signals have to be level shifted to the source of the high-side power device, which, in class D applications, swings between two rails of the circuit. The techniques presently used to perform this function are by either, transformer coupling or a high-voltage integrated circuit (HVIC) with a boot-strapped, high side driver. Either solution imposes a severe cost and performance penalty.
For transformer coupling, the transformer needs to have at least three isolated windings wound on a single core, adding to cost and space considerations. The windings need to be properly isolated to prevent breakdown due to the presence of high potential. Also, the gate""s drive circuit needs to be damped and clamped to prevent ringing between leakage inductors of the transformer and parasitic capacitors of switching MOSFETs.
In the case of high-voltage integrated circuits (HVIC), the HVIC has two isolated output buffers and logic circuitry which is sensitive to negative transients. The high-voltage process for the IC increases the size of the silicon die, and the boot-strap components add to the part count and costs. Such a system is also severely limited as to the switching frequency obtainable, which commonly is less than 100 KHz. Consequently, it uses the large sizes of EMI filters and resonant components aid requires larger space for implementation.
As described above, many types of dimming ballast circuits have been proposed that control the arc current of a fluorescent lamp. An external means of heating the cathode prevents sputtering of the cathodes whenever the arc current is reduced to less than 50% of its rated value. However, the efficiency of such dimming circuits falls off as the light intensity is reduced with the implementation of external cathode heating.
Multiple-ballast fluorescent lamp fixtures control lamps either in pairs or individually. For example, a three-lamp fixture may have a two-lamp ballast and a one-lamp ballast whereby the light intensity can be changed from 33% to 100% of its rated value by turning on a specific ballast. This requires at least two ballasts to cover a 3 to 1 range in brightness, which may increase cost and impact the available space in the fixture.
It is desirable to have a single ballast circuit for a multiple-lamp gas discharge fixture, wherein the ballast circuit is capable of powering a plurality of lamps from an efficiency perspective and from a packaging and cost perspective. Therefore, it is desirable for such a ballast circuit to be able to dim the light fixture by turning off individual lamps, leaving various combinations of lamps lit.
In one embodiment of the present invention, a ballast circuit comprises a plurality of inverters, each inverter configured for powering a load; and a controller operationally coupled to a shutdown control signal of each inverter for selectively shutting down any combination of inverters.
In another aspect of the invention, a dimming ballast is provided. The dimming ballast circuit comprises, a plurality of inverters, each inverter for powering a load; a controller operationally coupled to a shutdown control signal of each inverter for selectively disabling any combination of inverters to effectively disconnect the load associated with each disabled inverter.